1. Field of the Invention
The present invention relates to improvements to a gear shift controller for an automatic transmission, such as improved control of hydraulic fluid when the gear shift is executed.
2. Description of the Related Art
One way clutches have been used in automatic transmissions in order to avoid the complexity during a gear ratio change of disengaging one friction element and simultaneously engaging another friction element. Depending on whether the gear ratio change is an upshift or downshift, the one way clutch either releases or overruns during the period when the oncoming friction element is pressurized and engaged to complete the new driving connection among the components of the gear sets that produce the new gear ratio. If the one way clutch can be eliminated from an automatic transmission, its cost, weight, and size can also be eliminated.
A Japanese Patent Application First Publication No. Heisei 4-188572, published on Jun. 23, 1992, exemplifies a previously proposed gearshift controller for an automatic transmission in which the gear shift (clutch to clutch gear shift) can be actuated by a combination of an oncoming friction element and an offgoing friction element instead of a one way clutch.
In this Japanese Patent Application, the amount of hydraulic fluid pressure provided to these friction elements is provided on the basis of the speed of the input shaft of the automatic transmission. However, in the previously proposed gearshift controller for an automatic transmission, since the amount of hydraulic fluid pressure provided to these friction elements is provided on the basis of the input shaft speed, the hydraulic fluid pressure to the oncoming element and the offgoing element may be provided too early or too late due to a variation in input torque. When the change from the oncoming to the offgoing element occurs relatively late, the engine speed rises too high. When the change from the oncoming to the offgoing element occurs relatively early, engine speed reduction or torque reduction (commonly known as interlock) occurs.